This invention relates to a process for manufacturing a component such as an electrode for an electrochemical fuel cell that includes an article formed of flexible graphite sheet that is fluid permeable and has enhanced isotropy with respect to thermal and electrical conductivity. The graphite article has catalyst material selectively loaded thereon.
An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes denoted as anode and cathode surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. Often times, the electrodes also function as the gas diffusion layer (or GDL) of the fuel cell. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell is advantageously formed of a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal, or an alloy containing a platinum group metal, coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which joins with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen is removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
One limiting factor to the more widespread use of PEM fuel cells is the cost of the catalyst material. Metals such as platinum and the other platinum group metals are relatively expensive, especially as compared to the other elements of the cell, such as the graphite flow field plates. In conventional fuel cells, the catalyst material is bonded to the PEM or the electrodes in a non-selective manner. That is, the catalyst material is distributed relatively uniformly on the opposed surfaces of the PEM, with result that catalyst deployment is not as efficient as possible.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphite exhibits anisotropy because of its inherent structure and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Natural graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is at least about 80 times the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which can vary between up to about 10 times and as much as about 80 times or greater than that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles which generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
The present invention provides a process for manufacturing articles useful in a membrane electrode assembly for an electrochemical fuel cell comprising a pair of electrodes and an ion exchange membrane positioned between the electrodes, at least one of the electrodes being formed of a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels (or perforations) passing through the sheet between first and second opposed surfaces of the sheet, one of the opposed surfaces abutting the ion exchange membrane.
The transverse fluid channels, or perforations may be formed by mechanically impacting an opposed surface of the sheet to displace graphite within the sheet at predetermined locations. The transverse fluid channels are adjacently positioned and separated by walls of compressed expanded graphite.
In the operation of a PEM fuel cell the chemical reactions typically occur at specific places in the system. These reactions primarily occur at the interface of three components : the electrode (or gas diffusion layer), the membrane, and the catalyst. In the present invention, at least a portion of the walls of at least some of the transverse fluid channels have an adherent coating of activated carbon thereon or activated carbon distributed therein, the activated carbon loaded with catalyst.
More specifically, the process of the present invention selectively generates high surface area activated carbon that can be used as a catalyst support on the surface of an electrode or gas diffusion layer. A graphite sheet is perforated as described below and a resin such as a carbonizing phenolic or epoxy resin is applied to the surface. The resin is applied such that the channels or perforations are at least partially filled with the resin. Most preferably, all of the channels are filled with resin. At least a portion of the channels are filled.
The sheet is then cured and baked. The curing and baking shrinks the resin, and the resin adheres to the edges of the perforations. Also, because of the shrinkage the perforations reopen, allowing for the transport of fuel gasses and water. Most preferably, all the filed holes are reopened. At least a portion of the filled holed are reopened.
Preferably the portion of holes that are both filled and reopened is at least about 20%, more preferably greater than about 50%, and most preferably about 100% of the holes in the sheet.
The resin that is now selectively placed on the edges of and inside the perforations can be activated by various methods including those discussed below, producing a high surface area carbon attached to the edges and within the perforations. This high surface area carbon can then be loaded with a catalyst.
In one embodiment of the present invention, a method of manufacturing an electrode for an electrochemical fuel cell is disclosed, comprising providing a sheet of compressed mass of expanded graphite particles having a plurality of perforations defined by walls of the expanded graphite particles, and the perforations passing through the sheet between first and second opposed surfaces of the sheet; coating the sheet with a thermosettable organic resin, said coating step comprising filling a portion of said perforations with the thermosettable organic resin; curing and baking the sheet, and reopening a portion of the filled perforations during the curing and baking step; activating the thermosettable organic resin to form a high surface area carbon on the walls of the perforations; and loading a catalyst onto the high surface area carbon.
In another embodiment of the present invention, a method for manufacturing a component for a fuel cell is disclosed, the method comprising providing a sheet of a compressed mass of graphite particles having a plurality of transverse fluid channels having walls defined by the graphite particles and said transverse fluid channels passing through the sheet between first and second parallel, opposed surfaces of the sheet; filling a portion of said transverse fluid channels with a thermosettable resin; reopening said transverse fluid channels by curing and baking said sheet to selectively place the resin on the walls of a portion of the transverse fluid channels; activating said resin producing a high surface area carbon attached to a portion of the walls of the transverse fluid channels; and loading a catalyst to a portion of the high surface area carbon.